Integrated circuits are typically made from wafers of semiconductor material, such a silicon, germanium or gallium arsenide. The wafers are cut from cylindrical ingots, also called boules, of a single crystal of the semiconductor material. One method of making the single-crystal ingots is the Czochralski process of crystal growth that involves rotating a growing crystal that is pulled up from a “melt” of molten semiconductor material.
FIG. 1 (prior art) illustrates a single-crystal pulling device 10 used to perform a modified Czochralski process of pulling a single-crystal ingot. The ingot is made in a furnace 11 that includes a crucible 12 surrounded by a heater 13. A high-purity semiconductor material 14 is melted in crucible 12, which is typically made of quartz. A seed crystal is suspended from a rod and is dipped into the molten semiconductor material 14. As the semiconductor material crystallizes around the seed crystal, the rod is slowly pulled upward and is rotated around a vertical axis 15. Crucible 12 is rotated about vertical axis 15 in the opposite direction. A cylindrical, single-crystal ingot 16 is pulled from the melt.
The heating of the semiconductor material in the molten state causes convection currents 17 in crucible 12. The convection currents cause different areas of the molten semiconductor material below a crystallization disk 18 to have different temperatures. Molten semiconductor material at different temperatures crystallizes at different rates, resulting in defects in the crystal lattice structure of the semiconductor. For example, a large temperature gradient on crystallization disk 18 can cause both vacancies and interstitials in the crystal lattice structure, which result in lower quality wafers. By controlling the rate at which the crucible rotates and the rate at which the ingot is pulled up and rotated in the opposite direction, the temperature gradient is homogenized over the entire crystallization disk 18. By homogenizing the temperature gradient and thus the speed of crystallization, the profile of the crystallization interface between molten semiconductor material 14 and single-crystal ingot 16 is maintained as a smooth surface, and the crystal lattice structure is more uniform and free of defects. A consequence of rotating the growing single-crystal ingot is that ingot 16 assumes a cylindrical shape. Ingot 16 is then cut into wafers with a diamond saw and further polished to yield the semiconductor starting material for making integrated circuits.
The defects in the single crystal can be further reduced by suppressing the convection flows in the melt even more by using magnetic fields. FIG. 1 shows a cooling vessel 19 with a double-cylinder structure that houses two magnetic coils between inner and outer cylindrical walls. A first magnetic coil 20 is positioned opposite a second magnetic coil 21 with respect to vertical axis 15. The planes of both magnetic coils 20-21 are parallel to each other and to vertical axis 15. In addition to the rotation of crucible 12 and ingot 15 that homogenizes the temperature gradient of the molten semiconductor material 14, the magnetic fields generated by magnetic coils 20-21 suppress the movement of the convective flows and thereby further homogenize the temperature gradient of the molten semiconductor material 14 near crystallization disk 18.
FIG. 2 (prior art) shows the magnetic field lines 22 of the magnetic field generated between magnetic coils 20-21. Each field line represents the locations in the cross-sectional area between magnetic coils 20-21 that have the same magnetic flux density. FIG. 2 shows that there is a X-shaped cross section 23 of the magnetic field inside the crucible. Magnetic coils 20-21 homogenize the convective flows mainly by stirring them rather than by suppressing their flow. The stationary X-shaped field 23 crosses the convection currents in the rotating crucible 12 and stirs them.
FIG. 3 (prior art) shows a three-dimensional perspective view of one surface of equal flux density of a more uniform magnetic field 24 that can be generated by using four instead of two magnetic coils. Each pair of opposing coils is a Helmholtz pair oriented parallel to one another. FIG. 3 shows that only a small portion of the equal-flux surface of magnetic field 24 is close to being parallel to the crystallization plane, which is perpendicular to the z axis. Thus, rotating a crucible of molten semiconductor material within the magnetic field 24 depicted in FIG. 3 would tend to homogenize the convective flows by stirring them rather than by suppressing their flow. The molten semiconductor material immediately below the crystallization plane would be stirred as the molten semiconductor material rotating about the z axis crosses magnetic flux lines that are not parallel to the xy plane.
FIG. 4 (prior art) shows another single-crystal pulling device 25 as disclosed in U.S. Pat. No. 6,984,264. Pulling device 25 has four planar magnetic coils 26-29 that generate an even more uniform magnetic field than the field in FIG. 3. Each pair of opposing coils 26-27 and 28-29 are also Helmholtz coils oriented parallel to one another. But by positioning opposing pairs of magnets other than perpendicular to one another, pulling device 25 generates a more balanced and gradually varying magnetic field than the field of FIG. 3 or the X-shaped field of FIG. 2. For example, the angle between magnets 26 and 28 and between magnets 27 and 29 is 113 degrees.
But although pulling device 25 generates a larger area of uniform magnetic flux than the fields of FIGS. 2-3, the magnetic field generated by pulling device 25 is still not uniform enough in the plane of the crystallization disk between the ingot and the molten semiconductor material. The surfaces of equal flux density still fall sharply from the coils towards the middle. The flux lines shown in FIG. 4 represent a cross section in the xy plane of the magnetic field at a z coordinate of zero. FIG. 4 shows that only a small portion of the equal-flux surface of the magnetic field is parallel to the crystallization plane, which is perpendicular to the z axis. Thus, rotating a crucible of molten semiconductor material within the magnetic field depicted in FIG. 4 would tend to homogenize the convective flows by stirring them rather than by suppressing their flow. The molten semiconductor material immediately below the crystallization plane would be stirred as the molten semiconductor material rotating about the z axis crosses those magnetic flux lines that are not parallel to the xy plane.
A design for a pulling device is sought that can better suppress the convective flows of molten semiconductor material by generating a strong and more uniform magnetic field that is closer to being parallel to the crystallization plane between the molten semiconductor material and the ingot being pulled from the melt.